The invention relates to a method for calculating the resulting property combination of phase proportions and mechanical properties of a given alloy composition for a formable lightweight steel according to the preamble of patent claim 1.
Especially the hotly contested automobile market forces manufacturers to constantly seek solutions for lowering the fleet consumption while maintaining a highest possible comfort and passenger protection. Hereby on one hand the weight saving of a all vehicle components plays important role but also properties of the individual components that increase the passive safety of the passengers at high static and dynamic stress during operation and in the event of a crash.
In recent years significant advances have been made in the field of so-called lightweight steels, which are characterized by a low specific weight and at the same time high strengths and tenacity (for example EP 0 489 727 B1, EP 0 573 641 B1, DE 199 00199 A1) and a high ductility and are therefore of great interest for vehicle construction.
In these steels, which are austenitic in the starting state, the high proportion of alloy components with a specific weight far below the specific weight of iron (Mn, Si, Al) achieves a weight reduction, which is advantageous for the automobile industry while being able to maintain a conventional design.
From DE 10 2004 061 284 A1 for example a lightweight steel is known with an alloy composition (in weight %):
C0.04 to≤1.0Al0.05 to≤4.0Si0.05 to≤6.0Mn 9.0 to<18.0
remainder iron, inducing usual steel-accompanying elements. Optionally depending on the demand, Cr, Cu, Ti, Zr, V and Nb can be added.
This known lightweight steel has a partially stabilized γ solid solution microstructure with a defined stacking fault energy with a partially multiple TRIP-effect which transforms the tension- or expansion-induced transformation of a face-centered γ solid solution (austenite) into an ε-martensite (hexagonally densest spherical packing) and upon further deformation into a body-centered α martensite and residual austenite.
The high degree of deformation is achieved by TRIP—(Transformation Induced Plasticity) and TWIP—(Twinning Induced Plasticity) properties of the steel.
Many tests have revealed that in the complex interaction between Al, Si and Mn the carbon content is of paramount importance. Carbon on one hand increases the stacking fault energy and on the other hand widens the metastable austenite region. As a result the deformation-induced martensite formation and the strengthening associated therewith and also the ductility can be influenced.
It is also known that Mn and c are relatively strong austenite formers in contrast to Al Cr and Si, which are ferrite formers. A combination of these elements therefore leads to the formation of the two main phases austenite and ferrite and to further phases such as ordered ferrite phases and/or carbon based precipitations. These also play an important role for the mechanical technological properties of these steels.
Beside the influence on the formation of the microstructure phases an increasing proportion of Al and Si allows to further reduce the density of the steel/a problem however is that with increasing contents of Al or Si the casting with the known methods by macro segregations or bending of the strip or band during the solidification is more difficult or even impossible. Steel with Al— contents of >2% forms an oxide (Al2O3) during solidification at air which is extremely hard and brittle and thus makes casting and further processing difficult or even impossible. Thus process technical limits complicate the production of lightweight steels with ever-lower density significantly below the normal density of about 7.85 gr/cm3.
In addition the tests have revealed that lightweight steels are often already at small variations of the phase proportions of austenite and ferrite display great differences regarding strength at otherwise constant elongation and great differences regarding elongation at almost constant strength. Depending on the alloy composition, i.e., the interaction between austenite and ferrite formers, the phase proportions can hereby+ for example be between 5 and almost 100%, with strengths Rm between 600 and 1200 MPa, yield strengths Rp0.2 of 300 to 1120 MPa and elongation A80 between 5 and 40%.
The tests have also shown that different alloy compositions can lead to the same phase proportions of austenite and ferrite but nevertheless have very different mechanical properties. On the other hand lightweight steels with comparable mechanical properties may have very different phase proportions of austenite and ferrite.
However, due to the complex interactions between the individual alloy components it is still very difficult if not impossible to predict phase proportions and/or mechanical properties of these steels, so that materials with the demanded properties can only be determined by performing laborious and expensive tests.